If you're up to speed with system administration on other platforms, you might check out
UNIX and Linux System Administration Handbook, 4th Edition
, by Nemeth et al. It runs through how to perform many common tasks, pointing out how various UNIX and Linux releases differ from each other. Nice format. The 4th Edition covers AIX 6.1, so it's not 100% current, but it should be close enough for an intro. (As an aside, the 2nd Edition is useful for IRIX admins. Unfortunately, the IRIX-specific content was dropped in later editions.)